Solid state lightening devices, such as the well-known organic light emitting diode OLED, are widely used for a number of lighting applications. The OLED, when operated, provides light, having color point chromaticity coordinates within the standard CIE diagram.
The color point of an OLED device is affected by many factors related, for example, to the production process. This leads to a shift of the color point chromaticity coordinates with respect to an expected color point location in the CIE diagram. For example, different OLEDs being driven at the same current lead to different output lights having different color points.
Due to the sensitivity of the human eye to variations in chromaticity, it is desirable to minimize the color point variation. To prevent such color shifts, a pulse width modulation (PWM) method can be used to control the OLED lighting. Prior art (US 2007/0285378) proposes the mode on/off for controlling the driving current of the OLED. In this mode, the OLED is either full on or full off.